The proposal generally aims to develop a clear understanding of when and under what conditions pulmonary macrophages appear in prenatal lungs, what their antecedents are, what their capacity may be for respiratory defense, and how they relate to macrophage populations present in postnatal lungs. Stromal cells of the lungs are thought to provide a supportive microenvironment for macrophage development and will also be investigated. Current work has demonstrated the existence and ultrastructural appearance of a macrophage precursor in embryonic lung mesenchyme, designated CFU-rAM. The developmental potential of this cell is readily explored in lung organ cultures. Accordingly, plans are (1) to characterize CFU-rAM and its relationship to cells of macrophage lineage present in early embryos and older lungs; (2) to determine if its descendants are represented in the alveolar macrophage population of postnatal lungs; (3) to see if it may